Irradiating a target, such as in radiation treatment, requires a multiplicity of orientations between a target to be irradiated and a radiation beam. Typically, a radiation source is mounted on a gantry that rotates about a horizontal gantry axis. The radiation beam intersects, and is generally perpendicular to, the gantry axis. A patient is supported by a turntable that rotates the target about a vertical target axis, which theoretically intersects the gantry axis. Linear accelerators and cobalt therapy devices incorporate such gantry and turntable rotations to accomplish multi-orientation irradiation.
Since the beam is positioned by the gantry while the target is independently positioned by the table, the rotation axes of the gantry and turntable do not perfectly intersect with the radiation beam at the theoretical intersection point (isocenter). Rather, in the real world, there is a slight misalignment between the three, among other things due to the size and weight of the sagging rotating gantry and turntable. Consequently, an undesirable displacement or misalignment of the radiation beam occurs relative to the turntable.
Additional beam/target displacement may occur during treatment by target motion relative to the turntable, e.g., due to breathing. Patient support means in the prior art attempt to immobilize the patient's body relative to the turntable, but no such immobilization is described in the prior art for internal organ displacements relative to the immobilized body. Prior art methods, therefore, suffer from imprecise positional relationships between the table and the source compounded by uncontrolled target motion relative to the table. Beam/target displacements may be statically and dynamically compensated by on-line measurements and repositioning the target and/or the beam in a costly process known as Image Guided Radiation Therapy (IGRT).